This invention relates to an electric motor with a commutator and in particular to an oil migration barrier for such a motor.
In small, fractional and subfractional horsepower, permanent magnet direct current motors, brush gear and a commutator are typically used to transfer electrical power from motor terminals to rotor windings. The commutator, which consists of copper segments laid on a commutator base, is located adjacent one of the bearings rotationally supporting the rotor. Such bearings are typically oil impregnated sintered bushings due to wear and cost considerations. One disadvantage of these bearings is that the oil is not sealed within the bushing and tends to migrate along the shaft. Such migration if not checked, can extend to the commutator segments where the oil contaminates the brush/segment interface and mixes with brush dust ultimately rendering the motor inoperable by short circuiting commutator segments. Oil migration is also a problem for ball and roller bearings, albeit not as great, especially in high temperature applications such as within the engine compartment of a vehicle.
GB21922312 addresses this problem by providing a rubber oil collector disc which traps the oil. However, being rubber, care and space is required to prevent the disc from coming into direct contact with the bearing. Also, the motor is not suitable for use in very high temperature applications.
GB2207956 discloses a thin disc which is pressed onto the shaft so as to form a dish shape. This has the advantage of being very thin and thus, does not occupy significant shaft space but to work effectively, the motor must be rotating at a sufficient speed and for a sufficient period of time to fling any trapped oil from the surface of the disc, otherwise the oil migration is only delayed but not stopped. Also, effective sealing between the shaft and the disc is difficult to obtain and care is needed to ensure that the disc does not contact the bearing otherwise it may invert.
In both of these cases, separate spacers, which limit axial movement of the rotor through the bushings are required as well as axial space or shaft space for the oil stopper. Hence, as devices being driven by the motor are becoming smaller and lighter, smaller and lighter motors are desired. In some applications, the motor is operated under very low speed or very low angular distances and thus, do not generate the right conditions for an oil slinger to operate correctly/effectively.
Hence, there is a desire or need for a oil stopper which can withstand high temperatures and there is a further desire for an oil stopper which can withstand high temperatures and accept a degree of axial loading so as to provide a thrust surface for the brushing.